1. Field of the Invention
The present invention relates to a piezoelectric element used for a piezoelectric actuator and an injector using the piezoelectric element.
2. Description of the Related Art
In recent years, the piezoelectric element used as a piezoelectric actuator has begun to employ a laminated structure generally comprising 100 to 700 piezoelectric ceramic layers as thin as 20 to 200 μm and a plurality of internal electrode layers composed of a metal, which are stacked alternately in order to produce a large displacement with a low voltage.
The internal electrode layers each have an end portion thereof exposed to the side surface of the laminate, and there is the possibility of shorting between the internal electrode layers. Various countermeasures have been taken.
As an example, an organic material is arranged on the side surface of the laminate as an insulating structure. Although an organic material has an insulating effect in a normal air atmosphere, it is not effective in the case where the piezoelectric element is used in a highly humid atmosphere, or in a liquid such as kerosene or gasoline.
In a highly humid atmosphere, the organic material passes moisture, and in the case where a material of Ag group is used for the internal electrode layers, a migration, and hence shorting, occurs (Japanese Unexamined Patent Publication No. 3-12974). Kerosene or gasoline contains water and therefore provides the same problem as a highly humid atmosphere. Further, in the case where the interval between the internal electrode layers is extremely short (say, not more than 100 μm), a shorting, though not a migration, may occur under a high voltage. This is because the insulation resistance of the piezoelectric element is reduced by moisture absorption. A shorting occurs, therefore, even with the internal electrode layers composed not of Ag but a base metal such as copper or nickel.
In order to prevent the shorting, the piezoelectric element is usually built into a hermetic metal case (in dry air) (hereinafter referred to as the capsule structure). The capsule structure, however, increases the volume of the actuator as a whole and makes it difficult to reduce the size on the one hand and is accompanied with an increased number of parts resulting in an increased cost or a displacement loss. Further, the gap between the capsule and the piezoelectric element deteriorates the heat radiation characteristic.
A method for preventing shorting other than the capsule consists in using two or more layers of organic polymer films for insulation (Japanese Unexamined Patent Publication Nos. 62-88382 and 4-279070). Both of these methods are intended to prevent moisture transmission using an organic material having a high crosslink density on the outside. As far as an organic material is used, however, moisture is transmitted in some way or another, thereby unavoidably leading to shorting.
Still another method is to prevent moisture transmission by forming a metal film on an organic insulating material (Japanese Unexamined Patent Publication No. 2-130970). In the case where the organic insulating material develops cracking, however, a shorting occurs between the metal film and the internal electrode layers of the laminate.
An additional method consists in forming a film by superposing a plurality of inorganic insulating material films (Japanese Unexamined Patent Publication No. 3-155180). Since the inorganic material is hard, however, the displacement due the operation of the laminate cannot be absorbed and a cracks develop. The cracks thus developed allow moisture in. The same patent publication also describes a method in which the displacement is absorbed by an intentionally provided slit. In this method, however, moisture is transmitted by way of the slit in the same manner as through cracking.